It's actually pretty easy. Here is an excerpt from an article I wrote a while back:
The first step is to create a “Signature” brush is PS. You can scan in a handwritten signature or use a font you like. Here is how to do it:
1. Open PS and create a new file about 500x500 pixels. Make sure the background is set to “transparent” or "white" and while not critical, I use sRGB color space and 8-bit mode.
2. Select the “Type” tool and drag a box onto the transparent background. Select a type style you like and type your signature in black as you want it to appear on your images.
3. NOTE: I add the copyright symbol to the left of my name. To get the copyright symbol in windows, hold the “alt” key and type “0169” then release the alt key; on Mac, it's Option+G.
4. Once you have the signature the way you want it, merge the layers.
5. Now using the rectangular marquis, outline the signature area.
6. With the selection active, go to Edit/Define Brush Preset and give your brush a name you will recognize like “My Signature” and click “okay”. Your brush is now saved and is the very last brush in the brush palette dropdown list, but it is likely not the best size for your uses.
7. To correct the size you can now adjust it like any other brush, but the font can go fuzzy, so it's usually best to make a few brushes of different size brushes by re-sizing your font in step 2. I find 250 pixels to be a good standard size for prints, more like 30 for web. Simply size your brush to the new standard size you want a re-save it so you wont have to redo this step each time you use it.
8. For an actual signature replica, in lieu of step 2 above scan a copy of your signature, preferably black ink on white paper since you will be selecting the signature only for use as your brush. Select the signature with the magic wand tool and paste it into the transparent background. Proceed with steps 3 through 6.
Now that the brush is made, you can simply stamp it on an image, but with layers we can be far more creative. For example, I like to first select a complimentary color from my image for the signature itself. Then I like to place it on a layer above the final image that is slightly transparent with a drop-shadow – this gives the effect of the signature being done in a colored bead of water over the image.
BTW, I'll demo this in Moab
Cheers,